Fix `X-Feide-`-headers not cleared by auth proxy.
`string.sub(k, 8)` in Lua extracts every character from the eight byte onwards, and not the eight first bytes. Also, `x-feide` is 7 bytes, not 8. The result is that we only delete headers named something like `???????X-Feide`. This patch fixes this bug by correcting the `sub()`-call, and adding the trailing `-` to the header name, bringing the length to 8 characters.